Method for reducing the cost of voltage regulation circuitry in switch mode power supplies

ABSTRACT

A reduced cost voltage regulation circuit for switched mode power supplies. In one embodiment, a voltage regulation circuit includes a current sense circuit having a current sense terminal to conduct a current to be sensed by the current sense circuit. A voltage difference between the current sense terminal and a voltage reference terminal is substantially fixed when the current to be sensed by the current sense circuit is substantially equal to a first current sense threshold. The voltage regulation circuit also includes a first impedance coupled between the current sense terminal and the voltage reference terminal to provide a second current sense threshold. The second current sense threshold is equal to a sum of the first current sense threshold and a current to fow through the first impedance. The voltage regulation circuit further includes a second impedance coupled between the current sense terminal and an input terminal. The input terminal has a voltage threshold relative to the voltage reference terminal that is different from a voltage at the current sense terminal by an amount that is a product of the second impedance and the second current threshold.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of power supplies, and morespecifically to the regulation of power supplies. It involves a methodfor reducing the cost of regulation circuitry in switch mode powersupplies.

2. Background Information

Accurate regulation of power supplies is important in many areas. Forinstance, in devices employing sensitive electronic circuitry such ascomputers and televisions maintaining accurate power supply outputregulation is important to protect the electronic circuitry beingpowered from the output of the power supply, often referred to as thepower supply load.

Power supply regulation involves keeping either a current or voltagedelivered to a load within a specified range. A power supply is deemedto be in regulation if the load current or voltage is within thespecified range and is deemed to be out of regulation if the loadcurrent or voltage is outside the specified range.

Problems associated with conditions where regulation is lost or there isinstability in the power supply operation include damage to the load,improper load functioning, and the consumption of power by the load. Itis therefore desirable to regulate a power supply output withinspecified limits. Due to the cost sensitive nature of many applicationsemploying power supplies, it is also desirable to reduce the cost of thecircuitry used to maintain output regulation and stable operation ofpower supply.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention detailed illustrated by way of example and notlimitation in the accompanying Figures.

FIG. 1 is a schematic diagram of a buck converter power supply includingcircuitry to sense power supply output voltage involving the use of arelatively expensive optocoupler.

FIG. 2 is a schematic diagram of a buck converter power supply having arelatively high minimum power supply output voltage.

FIG. 3 is a schematic diagram of one embodiment of a low costnon-isolated buck converter power supply including a power supplycontroller in accordance with the teachings of the present invention.

FIG. 4 is a block diagram of one embodiment of a power supply controllerin accordance with the teachings of the present invention.

FIG. 5 is a schematic diagram of another embodiment of a non-isolatedbuck converter power supply including a power supply controller inaccordance with the teachings of the present invention.

FIG. 6 is a schematic diagram of yet another embodiment of anon-isolated buck converter power supply including a power supplycontroller in accordance with the teachings of the present invention.

FIG. 7 is a schematic diagram of still another embodiment of anon-isolated buck converter power supply including a power supplycontroller in accordance with the teachings of the present invention.

FIG. 8 is a schematic diagram of one embodiment of an isolated flybackconverter power supply including a power supply controller in accordancewith the teachings of the present invention.

FIG. 9 is a schematic diagram of another embodiment of an isolatedflyback converter power supply including a power supply controller inaccordance with the teachings of the present invention.

DETAILED DESCRIPTION

Embodiments of methods and apparatuses for reducing the cost ofregulation circuitry in switch mode power supplies are disclosed. In thefollowing description, numerous specific details are set forth in orderto provide a thorough understanding of the present invention. It will beapparent, however, to one having ordinary skill in the art that thespecific detail need not be employed to practice the present invention.In other instances, well-known materials or methods have not beendescribed in detail in order to avoid obscuring the present invention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, the appearances of thephrases “in one embodiment” or “in an embodiment” in various placesthroughout this specification are not necessarily all referring to thesame embodiment. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable manner in one or moreembodiments.

One commonly used topology of power supply converters is the buckconverter, an example of which is shown in FIG. 1. The detailedoperation of buck converters will be familiar to one skilled in the artand the following description therefore focuses on aspects relevant toembodiments of the present invention. The voltage regulation of thedirect current (DC) output 014 relative to rail 013 of the circuit inFIG. 1, is determined by a voltage regulation circuit comprising zenerdiode 012, optocoupler 010 and the internal circuitry of integratedcircuit 001 coupled to terminal 003. In one configuration, integratedcircuit 001 may be for example a TinySwitch power supply regulator ofPower Integrations, Inc., of San Jose, Calif., or another suitableequivalent. The voltage regulation circuit of FIG. 1 has a voltagethreshold determined by the breakdown voltage of zener diode 012 and theforward voltage of the optocoupler 010 LED 015.

In the configuration illustrated FIG. 1, the internal circuitry ofintegrated circuit 001 coupled to terminal 003, senses a current flowingthrough the transistor of optocoupler 010 to control the switching of asemiconductor switch internal to the integrated circuit 001. The controlof this internal semiconductor switch in turn controls the delivery ofenergy from the DC input terminal 007 to the DC output terminal 014 andhence the regulation of the output voltage of terminal 014 relative torail 013.

A disadvantage of using integrated circuit 001 as illustrated in FIG. 1is that the internal circuitry coupled to Enable terminal 003 sensescurrent flowing out of the Enable terminal 003, which forces users touse an optocoupler 010 as shown or other circuit configurations thatwill create current flow out of terminal 003, which increases theoverall circuit cost. It is desirable to reduce cost.

In another configuration, a pulse width modulator (PWM) controller, anintegrated circuit 101, is used in a buck converter topology, as shownin FIG. 2. In one configuration, integrated circuit 101 may be forexample a TopSwitch power supply regulator of Power Integrations, Inc.,of San Jose, Calif., or another suitable equivalent. In thisillustration, the voltage regulation of the DC output 114 relative tooutput terminal 116, is substantially determined by a voltage regulationcircuit comprising zener diode 112, resistor 111, capacitor 105 and theinternal circuitry of integrated circuit 101 coupled to terminal 102.Integrated circuit 101, includes a power transistor switch which isswitched on and off to control the delivery of energy from the powersupply input terminal 107 to the power supply output terminal 114.

During normal operation, integrated circuit 101 switches at a fixedfrequency. The fixed frequency defines a fixed switching cycle periodduring which the power transistor is on for an on time period and offfor the remaining time of the switching cycle period. The on time periodof the power transistor, as a percentage of the overall switching cycleperiod, is called the duty cycle.

During the on time period of each switching cycle period, the current ininductor 109 ramps up linearly. When integrated circuit 101 turns off atthe end of the on time period, catch diode 108 conducts to maintain theinstantaneous current in inductor 109 as will be known to one skilled inthe art. The source terminal 104 of integrated circuit 101 is thereforeclamped by catch diode 108 to a voltage that is the voltage of theoutput ground terminal 116 minus the forward voltage drop of catch diode108.

During this period, diode 110 rectifies the voltage across inductor 109.Since the forward voltage drop across diode 110 compensates the forwardvoltage drop of catch diode 108, the voltage across capacitor 113 issubstantially equal to the output voltage 114 relative to outputterminal 116. As such, node 115 forms an input terminal to the voltageregulator circuit comprising zener diode 112, resistor 111, capacitor105 and the internal circuitry of integrated circuit 101 coupled toterminal 102.

The operation of integrated circuit 101 is such that the duty cycle ofthe internal power transistor is substantially linearly reduced inresponse to an increase in the current flowing into control terminal102. As such the integrated circuit 101 receives feedback at controlterminal 102 via resistor 111 and zener 112 allowing the internal powertransistor duty cycle to be varied to maintain a substantially constantpower supply output voltage between terminals 114 and 116. The powersupply output voltage regulation threshold is therefore determined bythe choice of zener 112 and voltage of terminal 102.

Resistor 111 limits the current into the control terminal 102 withinspecified levels and typically has little influence on the voltageregulation threshold. Since the duty cycle of the power transistorinternal to integrated circuit 101 is varied substantially linearly inresponse to the current flowing into terminal 102, there is no fixedcurrent threshold associated with terminal 102 but instead an analogvariation of duty cycle in response to an analog current flowing intoterminal 102.

There are two main disadvantages of using integrated circuit 101 in abuck converter configuration as shown in FIG. 2. The first is thatterminal 102 has a voltage of approximately 5.8 volts relative to thesource or voltage reference terminal 104 which defines the minimum powersupply output voltage of at least 5.8 volts, when using a low cost powersupply configuration of the type shown in FIG. 2, which is not lowenough in many applications. Any modification of the circuit to allowlower output voltages typically adds significant complexity and cost tothe power supply. It is desirable to reduce cost.

A second disadvantage of using integrated circuit 101 as shown in FIG. 2is that since the integrated circuit 101 operation is substantiallyfixed frequency, power supply power consumption under light loadconditions is typically high since losses associated with switching theinternal power transistor are fixed. High power supply consumption underlight load conditions is no longer acceptable in many regions of theworld where stringent energy saving requirements have been put in placeby regulatory bodies.

FIG. 3 is a schematic of one embodiment of a power supply 300 includinga power supply controller 301, which overcomes shortcomings of the powersupply and power supply controllers discussed above. For instance, inthe illustrated embodiment, power supply controller 301 includes acurrent sense terminal 304 that has reverse logic compared to terminal003 of FIG. 1, thus allowing simplified feedback circuitry eliminatingthe need for the optocoupler 010 used in FIG. 1. This will reduce cost.Furthermore power supply controller 301 facilitates power supply designswith low output voltages at terminal 312 relative to terminal 314 sincethe voltage of terminal 304 relative to terminal 302 is lower than thevoltage of terminal 102 relative to terminal 104 in FIG. 2.

The power supply 300 in FIG. 3 is one embodiment of a non-isolated buckconverter power supply that utilizes power supply controller 301. In oneembodiment, power supply controller 301 includes four terminals: Bypassterminal 303, Enable or current sense terminal 304, Drain terminal 305and Source or voltage reference terminal 302. The Bypass terminal 303 iscoupled to a Bypass capacitor 315, which stores the charge required topower the power supply controller 301. The Drain terminal 305 is coupledto a power supply input voltage terminal 313, while the source terminal302 is coupled to the catch diode 311, inductor 309 and one terminal ofresistor 306.

In one embodiment, since the forward voltage drop across diode 307compensates the forward voltage drop of catch diode 311, the voltageacross capacitor 308 during the period that catch diode 311 isconducting is substantially equal to the output voltage 312 relative tothe voltage of terminal 314. Accordingly, in one embodiment, thepositive terminal of capacitor 308 may represent an “input terminal,”which in one embodiment has a voltage that is representative of orderived from the regulated output voltage of the power supply. Thecurrent sense terminal 304 is coupled to current sense circuitryinternal to the power supply controller 301, which is described in moredetail below with reference to FIG. 4. As will be discussed, theinternal current sense circuit has a current threshold. The voltage ofterminal 304 relative to voltage reference terminal 302 is substantiallyfixed when a current equal to the current threshold is conducted in thecurrent sense terminal 304.

In one embodiment, an output from the internal current sense circuitryis used to control the switching of a power transistor internal to powersupply controller 301, which controls the transfer of energy from thepower supply input terminal 313 to the power supply output terminal 312and hence regulates the output voltage of the power supply betweenterminals 312 and 314. In theory therefore the value of resistor 306 canbe chosen to determine the voltage across capacitor 308 at which thecurrent threshold above is reached and therefore regulate the outputvoltage of the power supply.

FIG. 4 shows one embodiment of an internal block diagram 400 of powersupply controller 301. Enable or current sense terminal 404 is coupledto a current sense circuit 417, which includes a transistor 415 coupledto a current mirror circuit 406, which is coupled to current source 407.In the embodiment shown in FIG. 4, current mirror circuit 406 mirrorsthe current flowing in transistor 415 substantially in a 1:1 ratio andcurrent sense circuit 417 therefore has a current thresholdsubstantially equal to the value of the current source 407 current. Inthe embodiment shown in FIG. 4, current source 407 has a currentsubstantially equal to 50 μA and the current threshold is thereforesubstantially equal to 50 μA.

In the embodiment shown in FIG. 4, the current sense circuit 417 has adigital output, which is the signal coupled to the input to logic gate416 from current source 407. In one embodiment, this digital output isin a first state when the voltage at the input terminal (e.g. positiveterminal of capacitor 308) is above a voltage threshold, and the digitaloutput is in a second state when the voltage at the input terminal isbelow the voltage threshold. In another embodiment, the digital outputis in a first state when the voltage at the input terminal is above thevoltage threshold by more than an upper hysteresis offset voltage, andthe digital output is in a second state when the voltage at the inputterminal is below the voltage threshold by more than a lower hysteresisoffset voltage.

In one embodiment, the hysteresis offset voltage at the input terminalis determined by a hysteresis in the current threshold of current sensecircuit 417. In one embodiment, the hysteresis in the current thresholdof current sense circuit 417 is determined by a hysteresis of currentsource 407 depending on the state of the digital output of circuit 417.The circuitry necessary to provide the hysteresis discussed above, isknown to one skilled in the art and is therefore not shown in FIG. 4 soas not to obscure the teachings of the present invention.

In the embodiment shown in FIG. 4, the state of the input to logic gate416 that is coupled to current source 407, determines, via other logiccircuitry which is not described so as not to obscure the teachings ofthe present invention, whether or not power transistor 413 is turned onwhen a clock output from oscillator 408 is received at another input tologic gate 416.

In one embodiment, the configuration of the current sense circuitry 417coupled to the current sense terminal 404 in FIG. 4 is such that thevoltage of the current sense terminal 404 relative to voltage referenceterminal 402, is substantially fixed when the current conducted at thatcurrent sense terminal 404 is substantially equal to the currentthreshold in accordance with the teachings of the present invention.This is true since the transistor 415 is turned on at a voltage governedby the voltage at its gate 405, which is fixed with respect to voltagereference terminal 402 at 1.7V−V_(TH), where V_(TH) is the turn onthreshold voltage of transistor 415, measured between the gate 405 andsource 404 of transistor 415. It is appreciated that the teachings ofthe present invention however are not tied to this specific embodimentillustrated in FIG. 4. Details such as the relative timing of logicsignals and the output of the current sense circuitry could vary whilststill benefiting from the teachings of the present invention.

In certain embodiments of circuits using the power supply controller400, it is an advantage to maintain the current threshold of currentsense circuitry 417 as low as possible. One embodiment is describedlater in FIG. 7 where the current flowing at current sense terminal 404is drawn from Bypass terminal 403. In one embodiment, Bypass terminal403 is supplied from an internal regulator circuit 411, which in turn iscoupled to be supplied from the Drain terminal 401. In the illustratedembodiment of FIG. 4, since the Drain terminal 401 typically has arelatively high voltage relative to voltage reference terminal 402, tomaintain low power consumption, it is beneficial to maintain a low valuefor the current threshold of current sense circuitry 417. As describedabove, in the embodiment shown in FIG. 4, the current sense threshold ofcurrent sense circuitry 417 is substantially equal to 50 μA. This valuefor the current sense threshold of current sense circuitry 417 imposespractical limitations on the operation of the circuit in FIG. 3, sincethe value of resistor 306 is very high. This high impedance at thecurrent sense terminal 304 introduces noise sensitivity on current senseterminal 304, which may result in unstable operation. However, thedesign of current sense terminal 404 allows a simple modification to thecircuit of FIG. 3, in accordance with the teachings of the presentinvention, to maintain stable operation whilst leaving power consumptionsubstantially unaltered, as described below.

FIG. 5 shows one embodiment of a DC/DC power supply circuit benefitingfrom the teachings of the present invention. This power supply circuit,in common with all subsequent power supply circuits described below, canalso be an alternating current (AC)/DC power supply with the addition ofsuitable rectification circuitry at the input to the power supply aswill be known to one skilled in the art. As can be observed, the powersupply circuit of FIG. 5 is similar to the power supply circuit of FIG.3 with an the additional resistor 506 coupled between current senseterminal 504 and voltage reference terminal 502. In the illustratedembodiment, the power supply controller 400 described in FIG. 4 may beutilized for power supply controller 501 and the current sense terminal504 is coupled to an internal current sense circuit having a currentthreshold.

In the illustrated embodiment, since the voltage of the current senseterminal 504 is substantially fixed relative to the voltage referenceterminal 502 when a current substantially equal to the current thresholdis flowing in current sense terminal 504, the current flowing inresistor 506 can be accurately determined. Therefore, this currentflowing in resistor 506 effectively introduces a second currentthreshold in accordance with the teachings of the present invention. Inone embodiment, this second current threshold is the sum of the currentthreshold of the internal current sense circuit coupled to current senseterminal 504 and the current flowing in resistor 506.

In one embodiment, since the forward voltage drop across diode 508compensates the forward voltage drop of catch diode 510, the voltageacross capacitor 512 during the period that catch diode 510 isconducting is substantially equal to the output voltage 513 relative tothe voltage of terminal 515. Accordingly, in one embodiment, thepositive terminal 517 of capacitor 512 may represent an input terminalrepresentative of or derived from the regulated output voltage of thepower supply.

In one embodiment, this second current threshold is utilized todetermine the choice of resistor value 507 to regulate the voltageacross capacitor 512. Since this second current threshold is greaterthan the current threshold of the internal current sense circuit coupledto current sense terminal 504, the value of 507 is lower than the valueof resistor 306 in FIG. 3. With a lower resistor value for resistor 507,noise sensitivity on current sense terminal 504 is therefore reduced,which improves stability in accordance with the teachings of the presentinvention.

It is appreciated that the power consumption of the power supply of FIG.5 is only slightly influenced by the addition of resistor 506 since thecurrent flowing in resistor 506 is sourced from capacitor 512, thevoltage across which is typically very low as it is derived from the lowvoltage output voltage of the power supply. Simply increasing thecurrent threshold of the internal current sense circuitry coupled to thecurrent sense terminal 504 of the power supply controller 501 would alsoallow a reduced value of resistor 507 to be used. However, as explainedabove with reference to FIG. 4, this can increase power consumption incertain circuit configurations, one of which will be described morefully in FIG. 7, which is undesirable.

FIG. 6 shows another embodiment of a power supply circuit benefitingfrom the teachings of the present invention. This circuit is similar tothat of FIG. 5 with the addition of a capacitor 608 in parallel toresistor 609. This forms an effective impedance 620, which replaces thesimple resistive impedance of resistor 507 in FIG. 5. This embodimentintroduces additional AC coupled feedback to current sense terminal 604,from the ripple voltage across capacitor 611, which can further improvecircuit stability in some power supply applications. The componentsymbol used to represent capacitor 608 in FIG. 6, is normally used torepresent an electrolytic capacitor as will be known to one skilled inthe art. It should be noted however that the benefit of capacitor 608can be realized using any type of capacitor. The addition of resistor607 coupled between current sense terminal 604 and voltage referenceterminal 602 again provides the advantages described above withreference to FIG. 5 by reducing the impedance at current sense terminal604 to reduce noise sensitivity at this terminal and therefore improvepower supply stability. In one embodiment, the addition of capacitor 608is not sufficient alone to provide this improvement and therefore stillrequires the addition of resistor 607 in accordance with the teachingsof the present invention.

FIG. 7 shows another embodiment of a power supply circuit, whichdemonstrates the value of the teachings of the present invention. In thepower supply circuit embodiment of FIG. 7, zener diode 710 andoptocoupler 707 are used to provide a very accurate reference, as isrequired in some applications, for the regulation of the power supplyoutput voltage 712 relative to power supply ground rail 714. Theoptocoupler transistor is coupled between a Bypass terminal 703 andcurrent sense terminal 704 of the power supply controller 701. In oneembodiment, the power supply controller 400 described in FIG. 4 may beutilized for power supply controller 701.

As will be understood to one skilled in the art, the gain of thefeedback loop in the embodiment illustrated in FIG. 7 is typically muchhigher than the circuit shown in FIG. 3 since the impedance of zenerdiode 710 above its breakdown voltage is much lower than the impedance306 in FIG. 3. As such the effective impedance at the current senseterminal 704 is lower in this circuit compared to that of current senseterminal 304 in FIG. 3 and the addition of a resistor from current senseterminal 704 to voltage reference terminal 702 is therefore notnecessary. Without the benefit of the teachings of the presentinvention, the current sense threshold of the current sense circuitry,internal to power supply controller 301, coupled to current senseterminal 304 would need to be increased to reduce impedance 306 in FIG.3. This would then compromise the circuit of FIG. 7, which does notrequire am increase in the current sense threshold of the current sensecircuitry, internal to power supply controller 701, coupled to currentsense terminal 704, by increasing the power consumption of the powersupply controller 701 and therefore the power consumption of the overallpower supply shown in FIG. 7.

FIG. 8 shows another embodiment of a power supply benefiting from theteachings of the present invention. The power supply of FIG. 8 is anembodiment of an isolated flyback power supply. As will be known to oneskilled in the art, the voltage across capacitor 810 is related to thevoltage across capacitor 812 through the turns ratio of transformer 807and is therefore a common solution employed to provide feedback toregulate the output of power supplies of the type shown in FIG. 8.

For the purposes of this description, terminal 820 is the input terminalto a voltage regulator circuit, which includes resistors 811 and 818. Inthe illustrated embodiment, the voltage at the input terminal 820 isrepresentative of the regulated output voltage of the power supply. Asshown in the illustrated embodiment, the power supply further includesvoltage reference terminal 802, current sense terminal 804 as well ascurrent sense circuitry, one embodiment of which is shown in FIG. 4 ascircuit 417, which is internal to power supply controller 801 andcoupled to current sense terminal 804. In one embodiment, the powersupply controller 400 described in FIG. 4 may be utilized for powersupply controller 801.

In accordance with the teachings of the present invention, resistor 818is coupled between the current sense terminal 804 and voltage referenceterminal 802 to provide a second current threshold equal to the sum ofthe current flowing through resistor 818 and the current threshold ofcurrent sense circuitry internal to power supply controller 801, oneembodiment of which is shown in FIG. 4 as circuit 417, coupled tocurrent sense terminal 804. In accordance with the teachings of thepresent invention, the voltage of the current sense terminal 804substantially fixed relative to the voltage reference terminal 802 whena current equal to the current threshold of the internal current sensecircuit flows in the current sense terminal 804. The resistor 811 iscoupled between the input terminal 820 and current sense terminal 804.The input terminal 820 has a voltage threshold relative to the voltagereference terminal 802 that is different from the voltage at the currentsense terminal 804 by an amount equal to the product of the value ofresistor 811 and the second current threshold.

FIG. 9 shows yet another embodiment of a circuit benefiting from theteachings of the present invention. This circuit is similar to that ofFIG. 8 with the addition of capacitor 919 in parallel to resistor 911.This parallel combination forms an effective impedance 921. In commonwith the parallel resistor and capacitor configuration of FIG. 6, thisconfiguration introduces additional AC coupled feedback to current senseterminal 904, which can further improve circuit stability in some powersupply applications. The addition of resistor 918 coupled betweencurrent sense terminal 904 and voltage reference terminal 902 againprovides the advantages described above with reference to FIG. 8 byreducing the impedance at current sense terminal 904 to reduce noisesensitivity at this terminal and therefore improve power supplystability. The addition of capacitor 919 is typically not sufficientalone to provide this improvement and therefore still requires theaddition of resistor 918 in accordance with the teachings of the presentinvention.

In the foregoing detailed description, the method and apparatus of thepresent invention have been described with reference to specificexemplary embodiments thereof. It will, however, be evident that variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the present invention. The presentspecification and figures are accordingly to be regarded as illustrativerather than restrictive.

What is claimed is:
 1. A voltage regulation circuit, comprising: acurrent sense circuit including a current sense terminal to conduct acurrent to be sensed by the current sense circuit, wherein a voltagedifference between the current sense terminal and a voltage referenceterminal is substantially fixed when the current to be sensed by thecurrent sense circuit is substantially equal to a first current sensethreshold; a first impedance coupled between the current sense terminaland the voltage reference terminal to provide a second current sensethreshold, wherein the second current sense threshold is equal to a sumof the first current sense threshold and a current to flow through thefirst impedance; and a second impedance coupled between the currentsense terminal and an input terminal of the voltage regulation circuit,wherein the input terminal has a voltage threshold relative to thevoltage reference terminal that is different from a voltage at thecurrent sense terminal by an amount that is a product of the secondimpedance and the second current threshold.
 2. The voltage regulationcircuit of claim 1 wherein the first impedance comprises a firstresistor.
 3. The voltage regulation circuit of claim 1 wherein thesecond impedance comprises a second resistor.
 4. The voltage regulationcircuit of claim 1 wherein the second impedance comprises a secondresistor coupled in parallel with a capacitor.
 5. The voltage regulationcircuit of claim 1 wherein the current sense circuit includes a digitaloutput, wherein the digital output is in a first state when the voltageat the input terminal is above the voltage threshold, wherein thedigital output is in a second state when the voltage at the inputterminal is below the voltage threshold.
 6. The voltage regulationcircuit of claim 1 wherein the current sense circuit includes a digitaloutput, wherein the digital output is in a first state when the voltageat the input terminal is above the voltage threshold by more than anupper hysteresis offset voltage, wherein the digital output is in asecond state when the voltage at the input terminal is below the voltagethreshold by more than a lower hysteresis offset voltage.
 7. The voltageregulation circuit of claim 1 wherein the voltage at the input terminalis representative of a voltage to be regulated by the voltage regulationcircuit.
 8. The voltage regulation circuit of claim 7 wherein thevoltage regulation circuit is included in a power supply circuit.
 9. Thevoltage regulation circuit of claim 8 wherein the voltage to beregulated is derived from at least one output of the power supplycircuit.
 10. The voltage regulation circuit of claim 8 wherein the powersupply is an AC/DC power supply.
 11. The voltage regulation circuit ofclaim 8 wherein the power supply is a DC/DC power supply.
 12. Thevoltage regulation circuit of claim 8 wherein the power supply is anisolated power supply.
 13. The voltage regulation circuit of claim 8wherein the power supply is a non-isolated power supply.
 14. The voltageregulation circuit of claim 1 wherein the current sense terminal and thevoltage reference terminal are terminals of an integrated circuit. 15.The voltage regulation circuit of claim 14 wherein the integratedcircuit further comprises a power transistor.
 16. The voltage regulationcircuit of claim 14 wherein the integrated circuit is a monolithicintegrated circuit.